Hydrogen or hydrogen-rich gases are produced on a large scale for use in the refining industry, and in the manufacture of ammonia, methanol and liquid hydrocarbons using the Fischer-Tropsch process. It is also used in numerous hydrogenation and petrochemical processes.
Hydrogen and the gases rich in hydrogen and carbon monoxide, a mixture known as syn (or synthesis) gas, are currently manufactured on an industrial scale, especially through steam reforming. The main reactions that take place in the steam reforming process are represented below (reactions 1, 2 and 3):CnHm+nH2O=nCO+(n+½n)H2 (Endothermic Reaction)  Reaction 1.CH4+H2O═CO+3H2 (Endothermic 206.4 kj/mol)  Reaction 2.CO+H2O═CO2+H2 (Exothermic, −41.2 kj/mol)  -Reaction 3.
The process of steam reforming may have diverse configurations, depending on the type of load and the desired use of the hydrogen-rich gas that will be produced. Such configurations may include a pre-reforming reactor, this option being particularly advantageous when the unit uses naphtha or different proportions of naphtha and natural gas as feedstock, when one wants to limit the amount of steam exported in the process, or when one would like to work with a low steam/carbon ratio. It is particularly useful when one wishes to obtain a hydrogen-rich stream with CO contents suitable for use in the manufacture of liquid hydrocarbons using the Fischer-Tropsch process.
The pre-reforming step is normally conducted in a fixed bed reactor with a nickel-based catalyst, under typical temperature conditions of between 350° C. and 550° C., a steam/carbon ratio of between 1 and 5, and a pressure of up to 40 bar.
The literature teaches us that nickel-based pre-reforming catalysts are subject to deactivation due to coke formation, which reduces catalyst activity and leads to high load losses in the industrial reactor.
Carbon may be formed on pre-reforming catalysts via one or more of the following reactions:CnHm=nC+(m/2)H2   Reaction 4.CH4═C+2H2   Reaction 5.2CO═C+CO2   Reaction 6.
According to the literature, carbon may be formed via hydrocarbon cracking reactions (reactions 4 and 5), and/or by CO disproportionation, depositing carbon atoms on the surface of the catalyst that are coke precursors, which in turn may encapsulate the active surface or produce carbon filaments on the metal particles. The formation of carbon filaments in particular can result in a large amount of carbon on the catalyst, potentially leading to its fragmentation. Both phenomena contribute to high levels of load loss in industrial reactors.
A known technique to reduce the problems associated with the phenomenon of coking of pre-reforming catalysts is to select the operating variables, in particular the steam/carbon and hydrogen/load ratios and the temperature, so that it is possible to operate at a temperature range such that, at temperatures above the maximum recommended temperature, coking known as filament or whisker coke occurs, and at temperatures below the minimum recommended temperature, coke is formed by the deposition of gum on the catalyst.
Although selecting the operating variables contributes to reducing the rate of formation of coke in pre-reforming reactors, the literature shows that coke accumulation is subject to the so-called kinetic regimen, where it can accumulate on the catalyst when its rate of formation exceeds the rate of gasification of the coke-forming species. Thus, coke may deposit on the catalyst even when the thermodynamic equilibrium would not predict its formation.
Factors that contribute to a higher rate of formation of coke on pre-reforming catalysts are known in the literature, and involve the use of low steam/carbon ratios, low hydrogen/load ratios, and the type of feedstock used. Considering feedstock, it is known in the state of the art that hydrocarbons in the naphtha range have a greater tendency to deposit carbon than natural gas. Among other factors contributing to coke formation, the literature shows that the presence of olefins is one of the most important factors, and can greatly accelerate the accumulation of coke on pre-reforming catalysts.
Techniques to reduce the effects of coke deactivation of pre-reforming catalysts are known to the state of the art.
U.S. Pat. No. 3,481,722 (1969, Engelhard) discloses a process for steam reforming a liquid hydrocarbon stream by contacting the hydrocarbon feed with steam and hydrogen with a catalyst containing a platinum group metal, at temperatures below 700° C. This solution however, of replacing nickel-based catalysts with a noble-metal based catalyst in the pre-reforming process, has high costs in terms of catalyst use, which limits its use for the large-scale manufacture of hydrogen or syn gas.
According to the literature, the use of alkali metals such as potassium in steam reforming or pre-reforming reduces the rate of coke deposit (Applied Catalysis A: General, 187 (1999) 127-140; Applied Catalysis, 287 (2004), 169-174). According to this disclosure, it is known that commercially available pre-reforming catalysts may contain varying amounts of alkali metals, such as potassium. The addition of alkaline compounds has an inconvenience in that they reduce the steam reforming activity of nickel-based catalysts, requiring that specific methods be adopted to obtain a steam reforming catalyst that incorporates alkali metals without harming their steam reforming activity, as disclosed in PI 1000656-7 A2 (2010 PETROBRAS).
Applied Catalysis, 31 (1987) pages 200-207 discloses a method to prepare a Ni/Al2O3—MgO—NiO type catalyst that has good resistance to carbon deposition, by coprecipitation. However, this catalyst is difficult to reduce, with a reduction level of around 50% at 500° C., which was associated with the formation of NiO—MgO solid solutions.
The literature shows that NiO—MgO phases can be formed that are only reduced when exposed to hydrogen at temperatures on the order of 800° C. to 850° C. (Applied Catalysis, 28 (1988) 365-377), temperatures that cannot be reached in a typical industrial pre-reforming reactor. This behavior results in the non-utilization of a significant phase of the Ni present in the catalyst, or the adoption of catalyst pre-reduction and passivation. It is known that commercially available pre-reforming catalysts containing magnesium in their formulation are often sold in the pre-reduced form, resulting in additional costs and handling precautions to avoid oxidation. It is also known in industrial practice that commercially available catalysts containing a free MgO phase must be carefully warmed in the absence of steam to avoid hydration of the MgO phase, which can lead to breaking the catalyst and an increase in load loss.
U.S. Pat. No. 7,427,388 (2008, Air Products) discloses a process for pre-reforming natural gas containing hydrocarbons larger than methane, which includes contacting steam, hydrogen and natural gas containing hydrocarbons larger than methane with a nickel-based catalyst, with an oxygen content that is less than the amount required to partially oxidize the hydrocarbons.
The addition of oxygen contributes to increasing the lifetime of the pre-reforming catalyst.
The addition of oxygen to the pre-reforming process implies additional manufacturing costs and/or the cost to purify the gas produced if air is used as the source of oxygen. There may also be technical limitations for its use in existing units, due to the high temperatures that may potentially exist in the process from the addition of oxygen.
According to the literature, olefins are a class of hydrocarbon that largely favor the formation of coke on steam pre-reforming or reforming catalysts.
In refineries, a typical stream containing olefin is refinery gas, which may be used to manufacture hydrogen or syn gas by steam reforming, so long as properly purified to remove sulfur compounds and olefins.
Olefin removal is normally performed in the pre-treatment section of the steam reforming unit, and typically involves the use of a reactor containing a NiMo/Alumina type catalyst, preferably in the sulfided state, and a high flow of recycled hydrogen, so as to provide the hydrogen required to saturate the olefins and control the reaction temperature due to the heat released by the exothermic olefin hydrogenation reactions. Such olefin removal processes, though efficient, are high cost in terms of the investment in the reactor, catalyst and compressors to recycle hydrogen.
Olefins may also be present in the tail gas of Fischer-Tropsch streams recycled to the pre-reforming section of the steam reforming unit. Typically, after undergoing a separation process for the removal of olefins and longer-chain oxygenated products, the gaseous effluent of a Fischer-Tropsch unit contains methane, ethane and carbon dioxide, in addition to unreacted hydrogen and carbon monoxide. However, often one observes olefins such as ethylene in this gas, due to the difficulty of purifying the stream using conventional techniques. As shown in the examples, commercially available pre-reforming catalysts are subject to rapid coking due to the presence of olefins in the feed, leading to a reduction in steam reforming activity and an increase in load loss in the pre-reforming reactor, and can lead to early pre-reforming reactor stops to replace the catalyst load.
Thus, although the technical literature contains numerous citations and descriptions, there is a need to provide a process that discloses the use of pre-reforming catalysts that resist coke deposition, especially for use with olefin containing streams.
The catalyst of the present invention may be used in the pre-reforming step of technologies to transform natural gas into syn gas in a first step, and then into liquid hydrocarbons, as an option for using the natural gas associated with crude oil in an FPSO (Floating Production Storage and Offloading) type rig. Such processes are currently being developed, with one of the challenges to be solved for their use on a larger, industrial scale being how to recycle streams containing light hydrocarbons formed in the Fischer-Tropsch section in the hydrogen producing section. Such recycling would enable a higher overall yield of the liquid products desired from the process in relation to the feedstock used, as well as adjusting the H2:CO ratio required for the “Fischer-Tropsch” section.
In practice, however, such recycling may compromise the lifetime of the catalyst in the pre-reforming section of the syn gas generation process, due to higher load losses due to coking, the result of olefins in the recycle stream. A technical solution that may be adopted is to replace the nickel-based catalyst of the pre-reforming section for producing hydrogen by a noble metal based catalyst. This solution, although it reduces any increase in load loss in the unit, has the inconvenience of employing a high cost, noble metal-based catalyst, making it difficult to enable the large scale, industrial use of this type of technology.
The present invention discloses the preparation of a nickel-based natural gas pre-reforming catalyst that is resistant to deactivation by coking due to the presence of olefins in the natural gas, or in the recycle gas from processes such as Fischer-Tropsch, which is added together with the steam and natural gas. Such a solution may be adopted to reduce the problems associated with high load losses during pre-reforming of olefin-containing gases, at a lower cost than the solution using noble metal based catalysts.
In addition to its use in technology to manufacture syn gas and Fischer-Tropsch products on an FPSO type rig, as an option for using the associated natural gas, the present invention may also be used in conventional processes for onshore manufacture of hydrogen and syn gas that include pre-reforming reactors in their process configuration, with the potential benefit of extending run times by reducing coking.